Process for producing a rotary anode and the anode produced by such process

ABSTRACT

The present invention is directed to an improved process for manufacturing a rotary anode for an x-ray tube, said rotary anode comprising a molybdenum support member on which a target layer consisting essentially of tungsten or a tungsten-rhenium alloy is provided by plasma spraying, the improvement comprising:
         a) preheating the support member to a temperature of from 1150° C. to 1600° C.,   b) placing the support member in a gaseous atmosphere containing hydrogen and having a pressure of from 0.5 to 0.9 bars and wherein hydrogen is present in a molar ratio of hydrogen to tungsten dioxide of from 5:1 to 50:1, and   c) plasma spraying the target layer onto the support layer in said gaseous atmosphere.
 
The invention is also directed to the anode produced by the process.

BACKGROUND OF THE INVENTION

Tungsten is used as a characteristic x-ray radiation generating material, for x-ray anode assemblies in CT scanners. Due to tungsten's high density and high atomic number, it is possible to produce gamma radiation with narrow frequency range. This explains why tungsten is the primary material of choice for x-ray generation. However, due to tungsten's density, and the stresses experienced during high rotational speed, tungsten is not the choice material of construction for the entire anode. Molybdenum (which has a high enthalpy, high melting point, lower density, and is easier to machine) is used as the base material for construction. The tungsten is then applied as a thin track along the surface of electron beam incidence to generate the x-radiation. Normally up to 10% by weight of rhenium is added to the tungsten to improve its malleability. Rhenium is less brittle than tungsten, and is the next element in the periodic system after tungsten, which explains its widespread use as an alloying element for tungsten.

There are three conventional methods for the application of the tungsten-rhenium track. The first method is via powder metallurgy, where the tungsten-rhenium powder is tape cast, slip cast, roll compacted, thermally sprayed, or waterfall processed into a track which is pressed, sintered and forged along with the molybdenum substrate (see U.S. Pat. No. 6,428,904). The '904 patent indicates that the process described therein improves the evenness of the surface and the interface between the track and the x-ray target substrate.

A second method describes the use of chemical vapor deposition (CVD) to apply the tungsten and molybdenum in a non-oxidizing atmosphere (see U.S. Pat. No. 4,461,020). The '020 patent indicates that this technique creates an improved bond and is an easily reproducible method.

The third and most prevalent method is plasma spray coating of the tungsten-rhenium track (see, e.g., U.S. Pat. Nos. 4,090,103, 4,390,368, 4,534,993 and 4,641,333). The '993 patent indicates that the plasma spray coating is carried out in a reduced pressure atmosphere of from 20 to 70 kPa. The '993 patent also indicates that in order to obtain optimum density, particle sizes of at most 45 μm be used.

U.S. Pat. No. 6,132,812 describes a plasma spray technique using inductive vacuum plasma spraying. The '812 patent indicates that the process described therein allows for increased residence time, which improves the ability of all particle sizes to melt and be deposited on the substrate in the molten state. It is also indicated that the process results in improved fatigue crack strength, and improved density.

All of these techniques fail to address causes for porosity, as well as the impact of oxygen on the density and propensity to degas.

DESCRIPTION OF THE INVENTION

The present invention is directed to an improved process for manufacturing a rotary anode for an x-ray tube, said rotary anode comprising a molybdenum support member on which a target layer consisting essentially of tungsten or a tungsten-rhenium alloy is provided by plasma spraying, the improvement comprising:

-   -   a) preheating the support member to a temperature of from         1150° C. to 1600° C.,     -   b) placing the support member in a gaseous atmosphere containing         hydrogen and having a pressure of from 0.5 to 0.9 bars and         wherein hydrogen is present in a molar ratio of hydrogen to         tungsten dioxide of from 5:1 to 50:1, and     -   c) plasma spraying the target layer onto the support layer in         said gaseous atmosphere.

The invention is also directed to the anode produced by the improved process.

The general method of manufacturing rotary anodes using a plasma spray coating technique is known in the art and is described in U.S. Pat. Nos. 4,090,103, 4,390,368, 4,534,993 and 4,641,333, the disclosures of which are herein incorporated by reference.

The use of hydrogen and a slightly reduced pressure will improve the density, adherence, and purity of the tungsten or tungsten-rhenium track. Hydrogen will serve to remove oxygen from the surfaces of tungsten, rhenium, and molybdenum. The oxygen removal will have two effects—first is to “activate” the surface of both the coating and the substrate improving adherence and secondly to remove oxygen prior to deposition resulting in an improved density and purity of the coating. The slightly reduced pressure will serve to remove the reactant oxygen bearing species. When referring to “slightly reduced pressure”, the closer the process is too atmospheric the more suppressed is the volatilization of tungsten oxide bearing species.

The determination of the amount of oxygen present in the tungsten or tungsten-rhenium alloy as tungsten dioxide is readily determined. Techniques for measuring the oxygen content of metal powders are known and include, for example, total x-ray fluorescence, secondary ion mass spectroscopy, x-ray photoelectron spectroscopy and Auger spectroscopy. Suitable analyzers are also available form LECO Corporation (TC400, TC500 and RO500C series). Based on the amount of oxygen measured, the actual amount of oxygen present in the form of the oxide present in the form of tungsten dioxide can then readily calculated The devices noted report the oxygen content as a % by weight per one gram sample. The moles of tungsten dioxide can then be calculated according to the following formula:

Mole of tungsten dioxide=[OC times WGT]÷32

where OC is the % by weight of oxygen per 1 gram sample, WGT is the total weight of the powder to be sprayed and 32 is the molecular weight of oxygen.

The molybdenum support member is preheated to a temperature of from 1150° C. to 1600° C. (preferably from 1300° C. to 1500° C.) and placed in a gaseous atmosphere containing hydrogen and having a pressure of from 0.5 to 0.9 bars (preferably from 0.7 to 0.9 bars) and wherein hydrogen is present in a molar ratio of hydrogen to tungsten dioxide of from 5:1 to 50:1 (preferably in a molar ratio of from 10:1 to 30:1). The tungsten or tungsten-rhenium alloy is the plasma sprayed onto the support layer.

Suitable devices for use in plasma spray coating known in the art and are commercially available. Such devices are included DC, arc and inductively coupled plasma devises. Such devices are commercially available from Progressive Technologies, Inc. (Michigan), Plasma Processes, Inc. (Alabama) and Tekna Plasma Systems Inc. (Canada).

As noted above, a hydrogen excess is required. This improves the reduction of WO₂. As the hydrogen to WO₂ molar ratio increases, the temperature at which complete reduction is possible decreases, and the temperature at which the volatilization becomes detectable increases. At a hydrogen to WO₂ molar ratio of 50:1, the temperature at which volatility of tungsten oxide species becomes detectable is almost 3000° C.; however, further temperature increase leads to increased volatility of elemental tungsten. At a hydrogen to WO₂ ratio of 75, no further increase in temperature for onset of volatility is noted, but tungsten metal evaporation increases.

In the presently claimed process, hydrogen is intentionally required to remove oxide species and activate the molybdenum substrate. The activation of molybdenum substrate and powder surfaces will also lead to an increase in the adherence of the tungsten or tungsten-rhenium alloy coating to the molybdenum substrate. The process effectively removes oxygen from all the metals and would increase the density of the coating. The process also will prevent any future degassing that would occur if oxide species were present in the coating.

Although illustrated and described herein with reference to certain specific embodiments, the present invention is not intended to be limited to the details described. Various modifications may be made within the scope and range of equivalents of the claims that follow without departing from the spirit of the invention. 

1. In a process for manufacturing a rotary anode for an x-ray tube, said rotary anode comprising a molybdenum support member on which a target layer consisting essentially of tungsten or a tungsten-rhenium alloy is provided by plasma spraying, the improvement comprising: a) preheating the support member to a temperature of from 1150° C. to 1600° C., b) placing the support member in a gaseous atmosphere containing hydrogen and having a pressure of from 0.5 to 0.9 bars and wherein hydrogen is present in a molar ratio of hydrogen to tungsten dioxide of from 5:1 to 50:1, and c) plasma spraying the target layer onto the support layer in said gaseous atmosphere.
 2. The process of claim 1, wherein the support member is preheated to a temperature of from 1300° C. to 1500° C.
 3. The process of claim 1, wherein said pressure is from 0.7 to 0.9 bars.
 4. The process of claim 1, wherein said ratio is from 10:1 to 30:1.
 5. The rotary anode produced according to the process of claim
 1. 